1. Field of the Invention
The present invention relates to projection devices in which an optical compensation sheet is arranged between a liquid crystal panel and a light polarizing plate.
2. Description of Related Art
FIGS. 8(a) and 8(b) are exploded oblique views of a liquid crystal panel 7 and light polarizing plates 73 and 74 that sandwich the liquid crystal panel 7. FIG. 8(a) shows a state in which no electric field is applied across the liquid crystal panel 7, whereas FIG. 8(b) shows a state in which an electric field is applied across the liquid crystal panel 7. As is well-known, each of the polarizing plates 73 and 74 allows the passage of only one of two polarized light components perpendicular to each other, and the polarizing plates 73 and 74 are arranged so that the oscillation planes of the polarized light components that are allowed to pass through them are 90 degrees apart from each other. The oscillation planes of the polarizing plates 73 and 74 must be exactly 90 degrees apart. In view of this, a configuration is known in which the polarizing plates 73 and 74 are rotationally adjusted within a plane perpendicular to the optical axis of the liquid crystal panel 7 (see Japanese Laid-Open Pat. App. Pub. No. 2000-39591).
The liquid crystal panel 7 is configured by enclosing liquid crystal molecules 57 between transparent display substrates 70 and 71. The two display substrates 70 and 71 are subjected to a rubbing process, and the rod-like liquid crystal molecules 57 are contained between the two display substrates 70 and 71 with the director of the molecules being twisted.
The liquid crystal panel 7 is of a so-called normally white type, in which, when no electric field is applied between the display substrates 70 and 71, the polarized light that has been transmitted through polarizing plate 73, the incident side, passes through polarizing plate 74, the output side, with its direction being bent 90 degrees due to the twist of the directors of the liquid crystal molecules 57, as shown in FIG. 8(a). As a consequence, the liquid crystal panel 7 appears bright, i.e., white.
As shown in FIG. 8(b), when an electric field is applied between the display substrates 70 and 71, the liquid crystal molecules 57 align vertically, and consequently the polarized light that has been transmitted through the incident-side polarizing plate 73 passes through the interstices between the liquid crystal molecules 57. The polarized light is blocked by the output-side polarizing plate 74, and thus the liquid crystal panel 7 appears black. The liquid crystal panel 7 displays an image by switching the presence/absence of the electric field in each of the microspaces.
It is known that in practice, however, in the liquid crystal panel 7, when an electric field is applied between the display substrates 70 and 71, the tilt angles of the liquid crystal molecules 57 continuously change along the thickness direction of the liquid crystal panel 7, as shown in FIG. 9. As a consequence, when an electric field is applied between the display substrates 70 and 71, light leakage occurs due to the birefringence of the liquid crystal molecules 57 in the vicinity of the substrates 70 and 71, causing contrast reduction, as is known.
In recent years, there has been a demand for greater image resolution. The devices that display such high-resolution images are required to maximize the contrast between black color and white color in projected images and project images clearly. If due to the birefringence of the liquid crystal molecules 57 just mentioned, the polarized light that should be blocked is transmitted through the liquid crystal panel 7 as indicated by the dot-dashed line in FIG. 8(b), black colors are not completely displayed black. Herein, “birefringence” indicates that the speed at which light propagates differs depending on the orientation of the plane in which it oscillates, and the orientation in which the speed is fast will be referred to as the “fast axis,” whereas the direction in which the speed is slow will be referred to as the “slow axis.”
In view of this, it has been suggested to provide optical compensation sheets 8 and 8a in which liquid crystal molecules 58 are arrayed along the thickness direction between the liquid crystal panel 7 and the incident/output-side polarizing plates 73 and 74, as shown in FIG. 9.
These are transparent sheets in which approximately disk-shaped liquid crystal molecules 58 composed of a discotic liquid crystal compound are aligned. The discotic liquid crystal compound is a chemical compound in which ester molecules are layered with benzene rings serving as cores.
The tilt angles of the liquid crystal molecules 58 change continuously along the thickness direction of the sheet, and the outermost liquid crystal molecules 58 are arranged substantially horizontally. As a result, the birefringence of the liquid crystal molecules 57 in the liquid crystal panel 7 is compensated, and thus light leaking from the liquid crystal panel 7 is not transmitted through the polarizing plate 74. Consequently, black colors can be reproduced completely black on the liquid crystal panel 7, maximizing the contrast.
These optical compensation sheets 8 and 8a are attached on to the polarizing plates 73 and 74 in many cases. The alignment orientation of the liquid crystal molecules 58 in the optical compensation sheets 8 and 8a needs to be parallel to the orientation of the display substrates 70 and 71.
The applicant, however, has found the following problems.
The optical compensation sheets 8 and 8a should be attached on to the polarizing plates 73 and 74 so that the alignment orientation of the liquid crystal molecules 58 is parallel to the orientation of the display substrates 70 and 71.
Nevertheless, there are cases in which the alignment orientation of the liquid crystal molecules 58 in the optical compensation sheets 8 and 8a does not turn out parallel to the orientation of the substrates because of error in attaching the optical compensation sheets 8 and 8a to the polarizing plates 73 and 74.
What is more, the optical compensation sheets 8 are cut out from a film sheet 85 into a desired dimension, but, as shown in FIG. 10, there are cases in which the cutting lines accidentally deviate, as indicated by the dotted lines, from the proper position.
In the optical compensations sheets 8 thus produced, the alignment orientation of the liquid crystal molecules 58 deviate from the originally intended orientation. As a result, light that must be blocked is allowed to pass through. Consequently, partial leakage of light occurs, into an area is essentially supposed to display black, causing display patchiness. Furthermore, since the optical compensation sheets 8 are affixed to the polarizing plates 73 and 74, if contrast reduction or display patchiness occurs it has been necessary to replace the optical compensation sheets 8 with different optical compensation sheets 8 having different optical axes. This makes it necessary to stock a variety of optical compensation sheets 8 having various optical axes, inviting adverse consequences such incidents of unneeded stock.
The applicant has conceived rotatively adjusting the alignment orientation of the liquid crystal molecules 58 in the optical compensation sheet 8 within a plane orthogonal to the optical axis to project uniform images having distinct contrast between black and white colors.